This invention relates to communication systems for sending information by transmission between stations over a limited band circuit such as, for example, a telephone line or telephone voice grade circuit. The invention provides for sending a plurality of separate messages each of which may occupy the available known bandwidth under normal conditions but by means of the present invention a plurality of such messages can be sent over a communication circuit which has a bandwidth approximately the same as that of the individual messages. In particular the invention is adapted to send a plurality of telephone voice messages over an ordinary dialed telephone line or circuit which has approximately a 3 kHz bandwidth.
There exist presently various digitized voice transmission systems which increase line utilization by data sample rates of 4800 baud for two simultaneous voice channels on a 9600 baud private line, but such systems involve substantial digital computer capability with the resultant high cost and impairment in quality of the voice signal, particularly with regard to speaker recognition.
Analog voice multiplexing is also employed for increasing communication line utilization through a technique known as Time Assigned Speech Interpolation (TASI) in which the various conversations are analyzed to insert the multiple messages for one channel in the gaps during pauses and between words found in the other messages. Generally this manipulation is done in digital form with the result reconverted to analog for transmission, thereby eliminating the need for modems or data sets. These systems require substantial data processing power and hence only become economically feasible when a large number of lines are processed.
Bandwidth reduction by speech compression in real time has been suggested for telephone line transmission as shown, for example, in the patent to French et al., U.S. Pat. No. 1,671,151. Such systems transform the original voice frequencies into a narrower spectrum thus permitting their transmission over a narrow band circuit.
Other arrangements for reducing the required bandwidth of the transmission circuit for voice signals employ a modulation and frequency offset demodulation technique to transfer a portion of the voice spectrum to a different portion of the speech band. In these schemes two or more portions of the speech spectrum are frequency offset to occupy the portion of the spectrum ordinarily occupied by a fractional part of the speech spectrum and these signals are sampled at an adequate rate and multiplexed in time before summing so that they can be applied to a narrowband transmission line and received by time demultiplexing. An example of such systems is found in the patent to Franco, U.S. Pat. No. 3,116,374, which uses multiplexing related to the number of subdivisions of the incoming signal spectrum. The patent to Harris et al., U.S. Pat. No. 4,314,104, utilizes what is termed the inherent time division multiplex character of the voice signal to accomplish a similar result. The patent to Morgan, U.S. Pat. No. 3,349,184, shows an arrangement for bandwidth compression and expansion by frequency division and multiplication and in connection with FIGS. 20-22 thereof discloses a five channel system with the subchannels proportioned to occupy a 3 kHz transmission bandwidth.